The present invention relates to a circuit in a panel display and, in particular, to a pixel driving circuit with threshold voltage and electroluminescent(EL) power compensation.
Active matrix organic light emitting diode (AMOLED) displays are currently emerging next generation of flat panel displays. As compared with an active matrix liquid crystal display (AMLCD), an AMOLED display has many advantages, such as higher contrast ratio, wider viewing angle, thinner module without backlight, low power consumption as well as low cost. Unlike an AMLCD display, which is driven by a voltage source, an AMOLED display requires a current source to drive an EL device. The brightness of the EL device is proportional to the current conducted thereby. Variations of current level have great impact on brightness uniformity of an AMOLED display. Thus, the quality of a pixel driving circuit is critical to display quality.
FIG. 1 illustrates a conventional 2T1C(2 transistors and 1 capacitor) circuit for each pixel in an AMOLED display. When a signal SCAN turns on a transistor M1, data shown as Vdata in the figure is loaded into a gate of a p-type transistor M2 and stored in the capacitor Cst. Thus, there will be a constant current driving the EL device to emit light. Typically, in an AMOLED, a current source is implemented by a P-type TFT(M2 in FIG. 1) gated by a data voltage Vdata and having source and drain connected to Vdd and the anode of the electroluminescent(EL) device, respectively, as shown in FIG. 1. The brightness of the EL device with respect to Vdata therefore has the following relation.
Brightness ∝ current ∝ (Vdd−Vdata−Vth)2 
where Vth is a threshold voltage of M2 and Vdd is a power supply voltage.
Since there is typically a variation of Vth for LTPS type TFT due to a low temperature polysilicon (LTPS) process, it is supposed that a non-uniformity problem in brightness exists in AMOLED display if Vth is not properly compensated. Moreover, a voltage drop on the power line also causes the brightness non-uniformity problem. To overcome such problems, implementation of a pixel driving circuit with Vth and Vdd compensation to improve display uniformity is desired.